The invention relates to an ultrasound imaging system comprising a transducer probe for supplying ultrasound waves to a subject area, for receiving ultrasound waves reflecting from the subject area, and for converting the reflecting waves into a first electrical signal, at least one position sensor provided in the transducer probe for detecting positional information on the transducer probe relative to the subject area during operation, and for generating a second electrical signal corresponding to the detected position, a processing unit for controlling the transducer probe and for processing the first and second electrical signals into an image.
Ultrasound imaging systems are commonly used to generate two-dimensional diagnostic images of internal features of a patients body. During operation the transducer is positioned on a surface of a subject area on a patients body and emits ultrasound waves. The ultrasound waves propagate into the subject area and are in part absorbed, dispersed, refracted and reflected by internal structures. The reflecting ultrasound waves are received back by the transducer probe, and are converted into electronic signals which are processed by the system into a two-dimensional image which can be used for diagnostic purposes.
Next to two-dimensional imaging, in recent years systems have been developed which are capable of three-dimensional ultrasound imaging. Three-dimensional imaging allows a better view of the organ being examined, and is for example especially suitable for visualization of a foetus and for early detection of tumors. To obtain such a three-dimensional ultrasound image, it is necessary to scan a volume instead of a two-dimensional plane. One way to realize this is to sequentially acquire two-dimensional slices of the subject area as described above, while detecting positional information on the transducer probe relative to the subject area by means of a position sensor during acquisition of the slices. This information on the changing position of the transducer during acquisition of the slices is then processed by the system into information on the position of each slice relative to the others, to combine the two-dimensional slices into a three-dimensional image. An ultrasound diagnostic imaging system as described in the opening paragraph is known from U.S. Pat. No. 5,127,409. In this system the position sensor comprises a number of ultrasound transducers for obtaining Doppler wave signals, which are provided in a part of the transducer probe which contacts the surface of the subject area during operation. The Doppler wave signals obtained by the ultrasound transducers are processed by the system into an estimation of the velocity of the transducer probe. By integrating the velocity over time, the translational and rotational characteristics of the position of the transducer probe relative to the subject area are then established.
A drawback of the known system is that the position sensor principally measures velocity. In order to obtain the positional information on the transducer probe, the velocity signals must be integrated over time. Any noise occurring during the period in which the transducer probe has been moving and the Doppler wave signals have been obtained, is also integrated which leads to large offset errors that cannot be easily predicted or corrected. This damages the accuracy of the positional information on the transducer probe and thus also the correct composition of the two-dimensional slices into a three-dimensional image.
It is an object of the invention to provide an improved ultrasound imaging system which offers more accurate position detection.
To achieve this object, an ultrasound imaging system according to the invention is characterized in that the position sensor comprises a unit for optically acquiring images of a surface of the subject area during operation, for acquiring information from said images, and for processing said information into positional information on the transducer probe relative to the subject area. By optically acquiring images of the surface of a subject area, being the skin surface of a patient, and thus acquiring information regarding the position of the transducer probe relative to the subject area, the acquisition of positional information is much less sensitive to noise occurring during movement of the transducer probe. The optical path between the scanned skin surface and the unit in the transducer probe is relatively short and is not easily disturbed. This enhances the accuracy of the detected position of the transducer probe and thus also the quality of the three-dimensional ultrasound image resulting from a composition of two-dimensional slices based on said positional information.
An embodiment of an ultrasound imaging system according to the invention is characterized in that the acquisition of information from said images comprises determination of a recognizable element in a first image, determination of the position of said element on the image, and mutual comparison of the positions of said element on further successively taken images. The information on the changes of the position of the element across successively taken images is relatively insensitive to noise and thus can advantageously be used to be processed into accurate positional information on the transducer probe relative to the subject area.
It is advantageous when the positional information on the transducer probe, which is provided by the unit, comprises translational and rotational information. The translational information is used by the system to determine the direction and the velocity of the transducer probe. When the direction of the transducer probe is known, the system has input on how to place the incoming two-dimensional slices with respect to each other in order to obtain a correct three-dimensional image. In this manner erroneous inversions are avoided. When the velocity of the transducer probe is known, the system is able to check whether this velocity is not too high. When the transducer probe is moved too fast across the subject area, certain slices are liable to be not properly scanned which would disturb the creation of a correct three-dimensional image. The rotational information is used by the system to determine whether any turns of the transducer probe occur which should be taken into account during the construction of a three-dimensional ultrasound image on the basis of the two-dimensional slices.
An embodiment of an ultrasound imaging system according to the invention is characterized in that the unit comprises illuminating elements, a lens system, an image sensor, and a digital signal processor. The illuminating elements provide the surface of the subject area with light, the light is scattered by the surface structure of the subject area, and this scattered light is focussed by the lens system. The image sensor takes repeated images of the pattern of light which is scattered from the surface of the subject area. By comparing successively taken images and determining the positions of identical regions of the scattered patterns on these images, the unit determines the positional changes of the transducer probe relative to the subject area during the scanning of the two-dimensional ultrasound slices. This information on the changing positions of the transducer probe relative to the subject area is then used by the system to construct a three-dimensional ultrasound image on the basis of the two-dimensional images of the slices. This unit can be manufactured against relatively low costs, and can be provided in the transducer probe in a relatively inexpensive manner. This benefits the cost-effectiveness of the whole system.
An embodiment of an ultrasound imaging system according to the invention is characterized in that at least two position sensors are provided. With two position sensors, the results of the measurement of translation of these sensors can be processed into an even more accurate determination of the rotation of the transducer probe relative to the subject area.
A further embodiment of an ultrasound imaging system according to the invention is characterized in that the processing unit is arranged to provide a warning signal dependent on the positional information. In this manner the system can warn the operator during handling of the transducer probe for any handling errors which might harm the acquisition of a correct three-dimensional image, such as a scanning velocity which is too high, or an unwanted degree of rotational movement of the transducer probe.